Moves to reduce the age at which people can watch hardcore pornography from 18 to 16 are to be backed by Charles Kennedy, the leader of the Liberal Democrats.

In a radical move which Kennedy argues revealed that the party was truly libertarian on social issues, the Liberal Democrat leader said he had no problem with proposals backing the change, to be discussed at this week's party conference.

'We are a liberal party,' he said in an interview with The Observer before the conference, which starts in Brighton today.

'We shouldn't be too apologetic or defensive about raising issues like this. No doubt there will be those who will roundly criticise us for the temerity to discuss these things, but it is a legitimate issue, people want to debate it.'

At present the law allows people over 18 to watch hardcore pornography or buy material from licensed sex shops. A motion to be debated at the conference on Tuesday will argue that the law should be relaxed and that a new R16 classification for pornographic films should replace R18.

The conference will be told the rules on pornography are full of 'abnormalities and contradictions' and that the law should better reflect that pornography is now made for women as well as men.

It will also be asked to back plans to allow for government regulation of the pornography industry and that it should be cheaper to set up sex shops.

'As a party we argue that you should lower the voting age from 18 to 16, we already accredit 16-year-olds with being able to sign up for the military, so it does seem inconsistent to say that they are not mature enough [to deal with this material],' Kennedy said. 'It is the logical application of the direction we are going in.'

He said he was well aware that family interest groups would condemn the move, but said the usual 'cacophony' around such subjects quickly dies.

'There is next to nobody in this country who would say that we should reverse legislation on homosexuality,' he said.

'We know that such a move was based on common sense and humanity, but that's not how it looked 30 years ago. We must be seen to be discussing these issues responsibly. I am sure the pornography debate will be extremely serious.'

Colleagues of Kennedy said his move, which will be viewed as controversial by many, revealed a new radical streak in a leader who is confident of his position.

The debate was abandoned last year because it was seen as too controversial to discuss at a conference that came in the direct wake of the 11 September attacks.

In his interview, Kennedy was even willing to suggest that the Lib Dems would one day be in government and that he could even hold the keys to Number 10.

'I certainly became leader of the Liberal Democrats because I wanted to see this party have a role in the governance of this country,' he said. 'Whether or not that would involve me being the Prime Minister or the Liberal Democrats being the predominant party, well, you can stare into the crystal ball on that one.

'The opportunity is there for us to be fully participating members in the Government. If there is this perception that the Conservatives are not going to win the next election and that the Government is looking more shop-soiled, then that has to be a tantalising prospect for us.'

Although the conference is likely to be overshadowed by the debate on Iraq, Kennedy said he was ready to argue passionately for new, decentralised public services that were more responsive to local needs.

He said he would back controversial local income taxes, which would give local authorities the power to raise or lower taxes by up to 3 per cent depending on the demands for funding schools and local services.

'You now have the devolutionary process under way in Scotland, Wales and Northern Ireland,' he said. 'The next stage of the argument has to be, if we are serious as a party about the decentralisation of power, that you get much more effective and efficient deployment of resources [when they are] locally focused rather than people sitting in Whitehall trying to second-guess everybody.'

He said all local authorities and any new regional assemblies in England would be given national baselines on spending, which they could not fall below. 'What we are arguing for is a system that is more flexible; what we are not arguing for is anarchy,' Kennedy said.

He said that since he married his long-term girlfriend Sarah Gurling this year he has had a different perspective on life, enjoying hill walking, golf and playing backgammon.

He admitted that he had taken up smoking again and that his weight was once again on an 'upward curve' after friends warned him he was taking dieting too seriously.

'These things are like cotton wool,' he said, looking at a pack of low tar cigarettes beside him in his office in the House of Commons.

'The tobacco firm should be paying me to smoke these things. I will get back off smoking again. But European history teaches you that you can only fight on one front at a time. Even Napoleon learned that lesson.'